I. Field of the Invention
This invention relates to a heat treatment process for shaped articles, particularly those suitable for use in the fabrication of automotive body panels. More particularly, the invention relates to such articles made from aluminum alloy sheet material that exhibits an improvement of hardness after painting and baking operations have been carried out.
II. Description of the Prior Art
Aluminum alloy sheet is being used more extensively nowadays as a structural and closure sheet material for vehicle bodies as automobile manufacturers strive for improved fuel economy by reducing vehicle weight. Traditionally, aluminum alloy is either direct chill cast to form ingots or continuous cast in the form of a thick strip material, and then hot rolled to a preliminary thickness. In a separate operation, the strip is cold rolled to the final thickness and wound into coil. The coil must then undergo solution heat treatment to allow strengthening of the formed panel during painting and baking (steps usually carried out on shaped automotive parts by vehicle manufacturers or othersxe2x80x94also referred to as the paint bake or paint cure).
Several aluminum alloys of the AA (Aluminum Association) 2000 and 6000 series are usually considered for automotive panel applications. The AA6000 series alloys contain magnesium and silicon, both with and without copper but, depending upon the Cu content, may be classified as AA2000 series alloys. These alloys are formable in the T4 or T4P temper conditions and become stronger after painting and baking. Good increases in strength after painting and baking are highly desirable so that thinner and therefore lighter panels may be employed.
It is highly desirable that the alloy sheet, when delivered to the manufacturer, be relatively easily deformable so that it can be stamped or formed into products of the required shapes without difficulty and without excessive springback. However, it is also desirable that the products, once formed and subjected to the normal painting and baking procedure, be relatively hard so that thin sheet can be employed and still provide good dent resistance.
To facilitate understanding, a brief explanation of the terminology used to describe alloy tempers may be in order at this stage. The temper referred to as T4 is well known (see, for example, Aluminum Standards and Data (1984), page 11, published by The Aluminum Association) and refers to alloy produced in the conventional manner, i.e. without intermediate batch annealing and pre-aging. This is the temper in which automotive sheet panels are normally delivered to parts manufacturers for forming into skin panels and the like. Material that has undergone an intermediate batch annealing, but no pre-aging, is said to have a T4A temper. An alloy that has only been solution heat-treated and artificially aged to peak strength is said to be in the T6 temper. Material that has undergone pre-aging but not intermediate batch annealing is said to have a T4P temper, and material that has undergone both intermediate annealing and pre-aging is said to have a T4PA temper. T8 temper designates an alloy that has been solution heat-treated, cold worked and then artificially aged. Artificial aging involves holding the alloy at elevated temperature(s) over a period of time. T8X temper refers to a T8 temper material that has been deformed in tension by 2% followed by a 30 minute treatment at 177xc2x0 C. to represent the forming plus paint baking treatment typically experienced by formed automotive panels.
An objective has been to provide a good xe2x80x9cpaint bake responsexe2x80x9d, i.e. a significant difference in hardness between the T4/T4P temper and the final T8X temper.
In the past, attention has been directed to steps carried out on the alloy sheets before the step of shaping the alloy sheets into products. For example, in U.S. Pat. No. 5,728,241 issued on Mar. 17, 1998 to Gupta et al., assigned to Alcan International Limited, a process of producing aluminum sheet of the 6000 series is described having T4 and T8X tempers that are desirable for the production of automotive parts. The aluminum alloy sheet material is subjected before shaping to solution heat treatment and quenching and then, before substantial age hardening has taken place, the sheet material is subjected to one or more heat treatments involving heating the material to a peak temperature in the range of 100 to 300xc2x0 C., holding the peak temperature for a period of time of less than one minute and then cooling the sheet material.
Similarly, in U.S. Pat. No. 5,616,189 issued on Apr. 1, 1997 to Jin et al., assigned to Alcan International Limited, a process is disclosed that involves subjecting a sheet product, after cold rolling, to a solutionizing treatment (heating to 500 to 570xc2x0 C.) followed by a quenching or cooling process involving carefully controlled cooling steps to bring about a degree of xe2x80x9cpre-aging.xe2x80x9d This procedure results in the formation of fine stable precipitate clusters that promote a fine, well dispersed precipitate structure during the paint/bake procedure to which automotive panels are subjected, and consequently a relatively high T8X temper.
While such approaches have met with success, they require modification of the traditional process for forming aluminum alloy sheet in strip form. This is inconvenient and may require expensive modification of existing fabrication equipment. Moreover, the disclosed processes involve rather careful temperature control that can be difficult or expensive to achieve.
It would be more convenient to be able to treat products made of aluminum alloy sheet at in some way after they have been formed into desired shapes. This is convenient because such products must anyway be handled and prepared for painting and baking, so additional steps at this point are easily arranged.
An object of the invention is to provide a process of producing a shaped article of enhanced hardness response without modification of a conventional procedure for produced aluminum sheet material in T4 or T4P temper.
Another object of the present invention is to provide a solution heat treated aluminum alloy product that exhibits a good hardness response during shaped article formation and finishing.
Yet another object of the invention is to produce a formed product from an aluminum alloy sheet material that has a low yield strength in T4 temper and a high yield strength in T8X temper.
According to one aspect of the invention, there is provided a process of producing a painted shaped article, comprising: obtaining a sheet article made of an aluminum alloy of the 2000 or 6000 series in a T4 or T4P temper; shaping the article to form a shaped article; subjecting the shaped article to a thermal spiking treatment involving heating the shaped article temporarily to a peak temperature in a range of 150 to 300xc2x0 C.; applying paint to the article to form a painted shaped article; and, if necessary to further enhance hardness of the painted shaped article and/or to cure the applied paint, baking the article at a temperature of at least about 177xc2x0 C.
The term xe2x80x9cthermal spike treatmentxe2x80x9d means a step in which the article is quickly raised in temperature from ambient (or other temperature at which the sheet material may be heated on the part treatment line) to a predetermined maximum temperature and is then quickly cooled or allowed to cool with or without providing a holding period at the peak temperature.
The term xe2x80x9cshaped articlexe2x80x9d includes any article obtained from sheet material for use in fabricating an article or component. The term may include a flat article simply cut from the sheet material, but often refers to a non-planar article produced by a bending or stamping step, e.g. for the production of an automobile fender or door. The term does not include unformed or uncut sheet material of indefinite length, e.g. coiled sheet produced directly from ingots or cast strip.
The present invention may be carried out with any precipitation hardening aluminum alloy of the AA2000 or AA6000 series, i.e. alloys containing Alxe2x80x94Mgxe2x80x94Si or Alxe2x80x94Mgxe2x80x94Sixe2x80x94Cu that are capable of exhibiting an age hardening response.
The invention also relates to a painted and shaped sheet article produced by the above process.
While it has been usual in the past to refer to the desired increase in hardness as the xe2x80x9cpaint bake responsexe2x80x9d, this term is becoming somewhat less appropriate as fabrication procedures advance. What is important is that this increase in hardness (the hardness response) occur between the shaping step (cutting/forming/stamping) initially carried out on the sheet form of the shaped product, and the finishing of the shaped product for delivery to the automobile manufacturer or the like.
In modern processes, there may not be a traditional paint bake step as paints of lower setting temperature may be employed. In the present application, the term xe2x80x9chardness responsexe2x80x9d will consequently be used instead of the more conventional term xe2x80x9cpaint bake response.xe2x80x9d This term refers to the change in tensile properties of the material at the end of a finishing process involving painting and optionally baking, compared to the properties prior to shaping. In the present invention, this increase may occur partially or fully during painting and baking, or partially or fully before such painting and baking, i.e. during the heat spike treatment itself, as will be explained more fully below.
The advantages of the invention, at least in preferred forms, include the following:
(1) The thermally spiked sheet material parts (e.g. automotive panels) acquire higher strength than those panels which have not been thermally spiked.
(2) In some forms of the invention, the maximum hardness response in the formed part can be obtained through a thermal spiking alone without relying on the paint cure process (or without providing a paint cure at all).
(3) The thermal spiking process, at least in some forms of the invention, can be performed on a continuous basis in ovens typically used for paint cure processes. The process therefore may be integrated seamlessly into the conventional shaping and finishing processes of parts formation, thus leading to convenience, efficiency and economy.
(4) The process provides an alternative possibility to acquire strengths higher than those obtained from the T4P material.